The behavior of ring laser angular rate sensors is well understood by those skilled in the art. Inherent in such sensors is the phenomena known as lock-in in which counter-propagating laser beams tend to lock together to a common frequency. The lock-in phenomenon causes performance errors which have deleterious effects in navigational systems.
To avoid or reduce the effects of lock-in, the laser angular rate sensor may be biased by dithering techniques such as those shown and described in U.S. Pat. No. 3,373,650 issued in the name of J. E. Killpatrick and assigned to the assignee of the present invention, and herein incorporated by reference. The biasing technique usually referred to as dithering may be implemented in a variety of ways including electro-optical and mechanical schemes. Since these biasing techniques directly affect the behavior of the counter-propagating laser beams, the sensor readout signal will contain not only rate information signals but also contain a signal component directly related to the dithering (alternating bias) of the sensor. This is true whether the readout is mounted directly on the sensor (block mounted) or off of the sensor (case mounted) like that shown in the aforementioned patent. The signal contribution in the readout signal due to dither is herein referred to as the dither signal component. For low noise navigational systems, the dither signal component in the readout signal usually must be minimized or removed to avoid control problems.
Prior art solutions to remove the dither signal component include, among others, notch filters. However, such notch filters generate gain and phase shift disturbances which can affect the stability of control loops. The desirable solution is to remove the dither signal component by generating a correction signal which is substantially equivalent to the dither signal component. This latter approach is taught in U.S. Pat. No. 4,344,706 issued to Ljung et al. Ljung teaches the use of a tracking circuit for tracking the clockwise and counterclockwise components of dither rotation. These dither components are subtracted from the usual readout signal which is responsive to the counter-propagating laser beams of the sensor thereby providing a corrected readout output signal.
Another approach is an electronic dither compensator as taught in U.S. Pat. No. 4,610,543, issued to Ferriss, and U.S. Pat. No. 4,344,706, issued to Ljung et al, both of these patents being incorporated herein by reference. These patents illustrate the up/down count technique of a gyro readout a mechanization for subtracting from these counts that contribution due to dither.
In the latter two mentioned patents, it is of paramount importance to recognize the ability to obtain a signal which is an exact replica of the dither signal component in the readout signal.